• Vlad Tereshenko, Udo Maierhofer, Laura A Hruby, Johanna Klepetko, Dominik C Dotzauer, Olga Politikou, Gregor Laengle, Matthias Luft, Christopher Festin, Roland Blumer, Konstantin D Bergmeister, and Oskar C Aszmann.
• 1Department of Plastic, Reconstructive and Aesthetic Surgery, Clinical Laboratory for Bionic Extremity Reconstruction, Medical University of Vienna, Austria.
• J. Neurosurg. 2023 Nov 1; 139 (5): 139614041396-1404.

ObjectiveIntrinsic function is indispensable for dexterous hand movements. Distal ulnar nerve defects can result in intrinsic muscle dysfunction and sensory deficits. Although the ulnar nerve's fascicular anatomy has been extensively studied, quantitative and topographic data on motor axons traveling within this nerve remain elusive.MethodsThe ulnar nerves of 14 heart-beating organ donors were evaluated. The motor branches to the flexor carpi ulnaris (FCU) and flexor digitorum profundus (FDP) muscles and the dorsal branch (DoBUN) as well as 3 segments of the ulnar nerve were harvested in 2-cm increments. Samples were subjected to double immunofluorescence staining using antibodies against choline acetyltransferase and neurofilament.ResultsSamples revealed more than 25,000 axons in the ulnar nerve at the forearm level, with a motor axon proportion of only 5%. The superficial and DoBUN showed high axon numbers of more than 21,000 and 9300, respectively. The axonal mapping of more than 1300 motor axons revealed an increasing motor/sensory ratio from the proximal ulnar nerve (1:20) to the deep branch of the ulnar nerve (1:7). The motor branches (FDP and FCU) showed that sensory axons outnumber motor axons by a ratio of 10:1.ConclusionsKnowledge of the detailed axonal architecture of the motor and sensory components of the human ulnar nerve is of the utmost importance for surgeons considering fascicular grafting or nerve transfer surgery. The low number of efferent axons in motor branches of the ulnar nerve and their distinct topographical distribution along the distal course of the nerve is indispensable information for modern nerve surgery.

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